It is proposed to construct and calibrate chloride, potassium and sodium liquid-ion exchanger microelectrodes for the purpose of measuring intracellular ionic activities in isolated segments of Amphiuma small intestine. In conjunction with short-circuiting techniques the ions being transported by the intestine can be determined and the probable site of transport by the absorptive cell (mucosal vs. serosal membrane) identified. Sites of passive ion transport will also be evident. The relative role of the chemical gradients for sodium and potassium and the electrical gradient (the mucosal membrane potential) in energizing active accumulation of labelled galactose will be determined at various luminal sodium concentrations. Measurement of cellular sodium and potassium concentrations by atomic absorption spectrophotometry will also allow determination of the relative amounts of free and "bound" cellular sodium and potassium. Finally, proximal, medial and distal small intestine will be compared as to the response of the mucosal membrane potential to various effectors of intestinal function.